1. Field of the Invention
The present invention relates to a phase change optical information recording medium capable of recording/reproducing information upon receiving light beam irradiation and, more particularly, to an optical information recording medium having two or more information recording layers accessible from one surface. The present invention also relates to an optical disk drive for irradiating the above optical information recording medium with a light beam to record information.
2. Description of the Related Art
A phase change optical recording medium capable of recording/reproducing information upon receiving light beam irradiation has been put into practical use as a storage medium having advantages such as a large capacity, high-speed access, and good portability. Its application purposes are expected to expand in future by increasing density.
Optical recording media are classified into a single-sided, single-layered type and a single-sided, n-layered type (n≧2). A single-sided, single-layered optical recording medium has only one recording layer accessible from one surface of the optical recording medium. That is, the single-sided, single-layered optical recording medium can record information on the single recording layer, erase information recorded on the recording layer, or reproduce information recorded on the recording layer by a light beam incident from one surface of the optical recording medium. To the contrary, a single-sided, n-layered optical recording medium has n recording layers accessible for one surface of the optical recording medium. That is, the single-sided, n-layered optical recording medium can record information on the n recording layers, erase information recorded on the n recording layers, or reproduce information recorded on the n recording layers by a light beam incident from one surface of the optical recording medium. The single-sided, n-layered optical recording medium can hold an information amount about n times that of the single-sided, single-layered optical recording medium.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2000-322770 discloses a technique related to a single-sided, n-layered optical recording medium.
A single-sided, two-layered optical recording medium has a first information layer and a second information layer sequentially from a side close to the light incident surface. The first information layer has a recording film and a dielectric protective film in contact with it sequentially from the side close to the light incident surface. Similarly, the second information layer has a recording film and a dielectric protective film in contact with it sequentially from the side close to the first information layer.
To record information on the second information layer, a laser power capable of melting the recording film of the second information layer is necessary. Hence, the transmittance of the first information layer must be designed to be relatively high. More specifically, it must be designed such that 50% or more of the laser power can reach the recording film of the second information layer.
A technique disclosed in ISOM (International Symp. on Optical Memory) 2000, Tech. Digest We-C-01 describes the structure of a first information layer. A transparent protective layer, interface layer, recording layer, interface layer, transparent protective layer, and metal reflecting layer are formed sequentially from the light incident side. The metal reflecting layer has an effect for causing the first information layer to have an appropriate reflectance value. However, it also absorbs light. For this reason, the optical power received by the second information layer decreases as compared to a case wherein no metal reflecting layer is formed.
In a phase change optical recording medium, information is recorded upon receiving light beam irradiation on a recording film. More specifically, when the recording film is irradiated with a light beam, the state of the recording film transits between a crystal state and an amorphous state to record information. The reflectance in a region in the crystal state is different from that in a region in the amorphous state. By detecting the difference, recorded information is read out.
By appropriately designing the dielectric film thicknesses on and above the recording film, the reflectance in the amorphous state is made higher than that in the crystal state, or the reflectance in the crystal state is made higher than that in the amorphous state. In the former case, the amorphous state corresponds to a high-level signal, and the crystal state corresponds to a low-level signal. Information is read out by making the high and low levels correspond to binary values and modulating the reflectance.
The first information layer preferably has a reflectance satisfying the following conditions. In a so-called High-to-Low case wherein a reflectance Ra in the amorphous state is lower than a reflectance Rc in the crystal state, the design is done such that Rc−Ra becomes as large as possible and Ra becomes as low as possible. The former condition is necessary to obtain a high reproduction signal intensity. The latter condition is necessary to reduce noise in the reproduction mode. In a so-called Low-to-High case wherein the reflectance Rc in the crystal state is lower than the reflectance Ra in the amorphous state, the design is done such that Ra−Rc becomes as large as possible and Rc becomes as low as possible. The effects of the respective conditions are the same as described above. That is, it is important to design such that the reflectance difference between the crystal state and the amorphous state becomes as large as possible, and the reflectance in the state corresponding to low level becomes as low as possible.
In designing the first information layer of the single-sided, two-layered optical recording medium, to meet the requirement to increase the transmittance, the degree of freedom of the reflectance Rc in the crystal state and the reflectance Ra in the amorphous state is considerably limited. For example, in Low-to-High, when a transmittance of 50% and Ra<4% are to be satisfied, Rc−Ra cannot be 8.5% or more. In High-to-Low, when Rc<4% is to be satisfied, Ra−Rc cannot be 7.5% or more. For this reason, the quality of a reproduction signal is low. Especially, it is difficult to reduce the error rate in high-density recording.
Particularly, when the interval between high and low levels is divided into multilevel values to execute multilevel recording, it is especially important that the interval between the high and low levels be sufficiently large, i.e., the reflectance difference between the crystal and amorphous-states be large. In the prior art, however, single-sided, two-layered recording and multilevel recording cannot be combined.